Electronic and software techniques associated with computer generated imagery are continuously evolving to provide increased resolution and greater clarity. The electronic and software improvements have at times outstripped the mechanical capabilities of recording devices and recording media to print this improved resolution and ' ' clarity. This situation presently exists in some applications in the field of thermal printing.
Many electronic and software techniques are used with thermal printers to control the flow of data to thermal print heads. See for example U.S. Pat. Nos. 4,745,413 (Scott Brownstein et al.) and 4,710,783 (Holden Caine et al.). These techniques create extremely accurate reproductions of successive lines of an image on a recording medium receiver. But, the creation of extremely accurate successive lines of an image does not necessarily create a complete image of equivalent clarity if the receiver is not transported across a print head with the same degree of accuracy.
A failure to achieve accurate spacing between successive lines of an image during its formation results in distortions of the image. For example, discrete horizontal stripes may appear in images that should otherwise have continuous tones. Some of these anomalies are acceptable in certain low resolution images having less than 150 lines per inch. In this low resolution range the presence of the discrete horizontal stripes is not easily discernible. However, when thermal printers are used to create images of higher resolution (e.g., 300 lines per inch), the need to control the distance between lines of the image becomes much more acute because the objectionable distortions become visually discernable.
One field of use for high resolution image recording by thermal printers is scientific applications such as applications still another problem adds to the acuity of line spacing control. The normal format of electron microscope image recording is on relatively small-sized paper (e.g., 5 inches .times.7 inches), This format facilitates the placement of recorded images into convenient record keeping books and the like. In the context of high resolution thermal printing operations on small sized receivers, it has been found that conventional techniques of loading, transporting and ejecting the receiver introduce variations in the speed of the receiver as it passes the print head. These variations manifest themselves as discernible horizontal stripes in the high resolution images and thus adversely effect the quality of the images.
It is possible to eliminate these deleterious effects of conventional receiver handling techniques by assuring that the printer is not engaged in image formation printer during the performance of any of the receiver handling steps. This is undesirable since it results in a thermally printed image being generated in a longer time than is realized by other recording techniques. Thus a need for a realistic speed of operation exists in high resolution thermal printers. Typically a printer is required to create a full-color image in less than one minute. A minute per print is not easily attainable if some receiver handling steps are not performed concurrently with the formation of an image.
It is desirable therefore to perform high resolution thermal printing with short print-cycle times and without introducing distortions in the printed image.